South Africa's first black President Nelson Mandela is being treated for a lung infection, the president's office has said.
This is the first time officials have revealed why Mr Mandela, 94, was rushed to a military hospital in the capital, Pretoria, on Saturday.
Tests showed a "recurrence of a previous lung infection", presidential spokesman Mac Maharaj said.
Mr Mandela is responding to his treatment, Mr Maharaj added.
News of the hospital stay has prompted much concern in South Africa.
The former president is regarded by most South Africans as the father of the nation, having inspired them to fight for democracy.
He led the struggle against white-minority rule before being elected the first black president in democratic elections in 1994.
Despite being imprisoned for 27 years by the apartheid government, after his release he forgave his former enemies and urged South Africans of all races to work together and seek reconciliation.
He was awarded the Nobel Peace Prize in 1993.
The BBC's Andrew Harding in Johannesburg says news that the frail Mr Mandela has another lung infection is likely to generate considerable anxiety.
The officials who have visited Mr Mandela in hospital since Saturday have all said he is doing well.
He was flown to hospital on Saturday from his home in Qunu village in Eastern Cape province by the military, which is responsible for his healthcare.
Local media reported that the decision to move him was taken so quickly that some family members and his own foundation were initially unaware of it.
But Mr Maharaj has repeatedly said that Mr Mandela is doing well in hospital and there is no cause for alarm.
Mr Mandela was last admitted to hospital in February when he was treated for abdominal pain.
In January 2011, he was treated for a serious chest infection.
While in prison in the 1980s, Mr Mandela was also diagnosed with tuberculosis - his lungs are said to have been damaged when he worked in a prison quarry.
Mr Mandela retired from public life in 2004 and has been rarely seen in public since.